Framework Oxygen Atoms Research Articles

Ion-exchange in natural zeolite stilbite and significance in defluoridation ability

A preliminary inspection of natural zeolites samples in the north east of Ethiopia showed a large deposit of highly pure stilbite (hereafter called |CaNa|-STI). Rietveld refinement of synchrotron powder diffraction data (C2/m, a=13.62(1)Å, b=18.2563(1)Å, c=11.28(1)Å, β=127.8(2)°) has shown that both Ca2+ and Na+ ions reside at the intersection of the ten- and eight-ring channels, and are completely surrounded by H2O molecules that make close contacts with the zeolite framework. Therefore, no coordination of the cations with framework oxygen atoms is observed. In an attempt to improve the defluoridation capacity, |CaNa|-STI was subjected to a NH4+ and subsequent Ca2+ ion-exchange treatment. Analysis of the diffraction pattern of the |Ca–NH4|-STI sample revealed the presence of a mixture of crystals with monoclinic and orthorhombic symmetry. This double-exchange treatment notably improved the ability of this material to remove fluoride from water, what is probably related to the formation of connectivity defects that are generated during the exchange treatments.

Introducing copper ions into zeolite Y by the thallous ion exchange method: single crystal structure of |Cu21.6Tl39.2|[Si121Al71O384]–FAU

The thallous ion exchange (TIE) method was used for the first time in an attempt to introduce copper ions into zeolite Y (FAU, Si/Al = 1.69). |Cu 10.9 + Cu 10.7 2+ Tl 39.2 + |[Si121Al71O384]–FAU was prepared by reacting fully dehydrated and fully Tl+-exchanged zeolite Y (Tl71–Y) with CuCl2(g) and its decomposition products CuCl(g) and Cl2(g) at 673 K under anhydrous conditions. Its structure was determined using single-crystal crystallography with synchrotron X-radiation and was refined in the space group $${Fd}\bar{3}$$ m (a = 24.769(1) A) with all 903 unique data; the final error index, R 1 = 0.075, was calculated using only the 858 reflections with F o > 4σ(F o). About 45 % of the Tl+ ions were replaced by 21.6 copper ions per unit cell at the following sites (distances to nearest framework oxygen atoms are given): 10.7 Cu2+ at site I′ in the sodalite cavity opposite double 6-rings (Cu2+–O = 2.093(9) A), 3.5 Cu+ at site II opposite single 6-rings in the supercage (Cu+–O = 2.24(3) A), and 7.4 Cu+ at site III near 12-rings in the supercage (Cu+–O = 2.45(7) A). All Cu+ ions are in supercages where they are easily accessible to guest molecules. The remaining ca. 39 Tl+ ions per unit cell occupy three distinct positions: 12 are at a second site I′ (Tl+–O = 2.571(9) A), 23 are at a second site II (Tl+–O = 2.732(10) A), and 4 are at site III′ (Tl+–O = 2.871(16) A) near triple 4-rings in the supercages.

A theoretical study of Cu clusters in siliceous erionite

In terms of periodic density functional theory method, we present a theoretical study of the structure and electronic properties of different composites, obtained by the inclusion of distinct Cux (x=2,4,6,8) clusters within the C1 cage of siliceous erionite–zeolite. Results shows that for a periodic distribution of Cux clusters on zeolite, the hosting of those is trough electrostatic confinement which is governed by framework oxygen atoms located at particular equivalent sites. However, when the permanency of a cluster in a C1 cage is compromised, the cluster–cluster long range Coulomb interactions could rise to keep it inside. In addition, results also show that the electronic properties of free Cux clusters are largely transferred to the formed composite.

Using CuCl vapor to ion exchange copper into zeolite Na–Y. Single crystal structure of |Cu30Na30Cl9|[Si121Al71O384]–FAU containing [formula omitted], Cu4Cl7+, Cu3Cl2+, and Cu2+

A single crystal of zeolite Na–Y (FAU, Si/Al=1.69) was treated with 1.6Pa of anhydrous CuCl vapor at 623K to yield Cu30Na30Cl9–Y, in more detail ∣Na+30Cu2+2(Cu16Cl721+)0.44(Cu4Cl7+)3.2(Cu3Cl2+)2.8|[Si121Al71O384]–FAU. Its structure was determined crystallographically with synchrotron X-radiation and was refined in the space group Fd3¯m (a=24.644(1)Å) with all 1159 unique data; the final error index, R1=0.064, was calculated using only the 1028 reflections with Fo>4σ(Fo). About 58% of the Na+ ions in the zeolite were replaced by Cu2+, Cu3Cl2+, Cu4Cl7+, and Cu16Cl721+. Cu4Cl7+ clusters center 41% of the sodalite cavities. Each is a centered tetrahedron with Cu–Cl=2.931(3)Å; each terminal Cu2+ ion bonds to three framework oxygen atoms of one ring of a double 6-ring with Cu–O=2.222(4)Å and O–Cu–O=110.8(5)°. Cu16Cl721+ clusters center 5.5% of the supercages. Four Cu+ ions coordinate tetrahedrally to the chloride ion at its center (Cu–Cl=2.66(11)Å). Each pair of these Cu+ ions is bridged by a Cl− ion (Cu–Cl=3.03(7)Å) along each tetrahedral edge. Each of these Cl− ions then coordinates to two Cu2+ ions (Cu–Cl=2.58(7)Å), each of which bonds to two 12-ring oxygen atoms (Cu–O=2.35(3) and 2.41(4)Å with O–Cu–O=67.8(8)°). About 17.5% of the 12-rings are centered by near trigonal planar Cu3Cl2+ clusters (Cl− at the center, Cu–Cl–Cu=117.2(3)°, and Cu–O=2.52(4)Å). The single 6-rings are fully occupied by the remaining 2.0(5) Cu2+ ions (Cu–O=2.158(7)Å) and the 30.0(5) Na+ ions (Na–O=2.297(4) Å) per unit cell. Most of the Cu+ and Cu2+ ions in this material are accessible to guest molecules via the 12-ring channel system.

The role of rare earth elements in zeolites and cracking catalysts

This brief review has the objective of showing the role of the rare earth in the context of Fluid Catalytic Cracking (FCC). REHY and RENaY zeolites, two of the most important components of FCC catalysts, were extensively studied in the past years. Their properties were evaluated in different conditions and the discussions of their main characteristics were revisited. It has been shown that RE elements migrate to smaller cages upon calcination and, once located in these cages, they form bridges with framework oxygen atoms, stabilising the zeolite structure. Some hydrolysis reactions take place over RE cations, generating Brønsted acidity and the higher the ionic radius of the RE atom, the higher the degree of hydrolysis. Besides, a correlation between the ionic radius of the RE atom, the Brønsted acidity of the zeolite and the rate of hydrogen transfer reaction was observed. Also, the use of rare earth elements as a vanadium trap has been analysed. Finally, new applications of rate earth containing zeolites in other fields than FCC catalysis indicate that this subject is still a matter of great interest.

Cation site preference in zeolite clinoptilolite: A density functional study

The structure of zeolite clinoptilolite and the coordination of extraframework cations (Ca2+, Ba2+, Na+ and K+) by framework oxygen atoms are examined by density functional theory (DFT) using two methods: the ONIOM two-layer model (DFT/MM) and periodic DFT. Both ONIOM and periodic model calculations predict correctly the clinoptilolite structure and find configurations with Al → Si substitution at the T2 site which interconnects the xz layers via its apical oxygen atom as the most stable ones, in agreement with experimental data. The configurations with Al → Si substitution at the T1 sites are favorable for migration of cations into the large channel A from the eight-member rings which are side rings of channel A and opened in channel C (c-rings); the process is energetically most favorable for the Na+ cations. K+ and Ba2+ cations occupy sites in the vicinity of the c-rings: M1, M1a and M3, among which K+ prefers the M3 site inside the c-rings. The molecular electrostatic potential (MEP) maps reveal areas of increased nucleophilic properties inside the large channel A, to which cations can migrate upon heating, or incoming cations can be retained. The minimal interatomic distances between two equivalent cations, residing in channel A are 4.850Å (Ca2+–Ca2+); 5.025Å (Na+–Na+) and 4.305Å (K+–K+). The ONIOM method should be preferred over periodic models for describing cations in the c-rings: the smaller c-parameter of the unit cell replicates the cations in adjacent rings along the [001] direction.

Direct evidence of the role of dispersed ceria on the activation of oxygen in NaX zeolite by coupling the 17O/16O isotopic exchange and 17O solid-state NMR

The possibility to understand the phenomena of oxygen diffusion in the solids may be decisive to explain the role of framework oxygen atoms in a catalytic mechanism or in catalyst regeneration steps. In this work, two highly efficient characterization techniques have been associated: the 17O/16O isotopic exchange and the 17O high field solid-state NMR spectroscopy. This combination leads to a powerful tool to investigate the interaction between gaseous dioxygen molecule and lattice oxygen atoms. Here, it permits to demonstrate the enhancement of the participation of zeolitic oxygen atoms in the exchange with the gas phase when ceria is simply mixed with NaX.

Mechanism of Framework Oxygen Exchange in Fe‐Zeolites: A Combined DFT and Mass Spectrometry Study

The role of framework oxygen atoms in N(2)O decomposition [N(2)O(g)→N(2)(g) and 1/2O(2)(g)] over Fe-ferrierite is investigated employing a combined experimental (N(2)(18)O decomposition in batch experiments followed by mass spectroscopy measurements) and theoretical (density functional theory calculations) approach. The occurrence of the isotope exchange indicates that framework oxygen atoms are involved in the N(2)O decomposition catalyzed by Fe-ferrierite. Our study, using an Fe-ferrierite sample with iron exclusively present as Fe(II) cations accommodated in the cationic sites, shows that the mobility of framework oxygen atoms in the temperature range: 553 to 593 K is limited to the four framework oxygen atoms of the two AlO(4)(-) tetrahedra forming cationic sites that accomodate Fe(II). They exchange with the Fe extra-framework (18)O atom originating from the decomposed N(2)(18)O. We found, using DFT calculations, that O(2) molecules facilitate the oxygen exchange. However, the corresponding calculated energy barrier of 87 kcal mol(-1) is still very high and it is higher than the assumed experimental value based on the occurrence of the sluggish oxygen exchange at 553 K.

Theoretical modeling of optical properties of Ag8 and Ag14 silver clusters embedded in an LTA sodalite zeolite cavity

Optical properties of silver Ag(n) nanoclusters are demonstrated to be dependent on their size, structure and charge state. It is found that when being contained in the sodalite cavity of LTA zeolite the tetradecanuclear hexacation silver cluster Ag14(6+) is stable. Its lower-lying states and optical spectrum are theoretically determined using the quantum chemical TD-DFT method. Its ground state possesses an outer-shell electron configuration of A1g(2)T2g(6) mimicking the s(2)p(6) valence of noble gas atoms. These frontier orbitals are constructed from 5s,5p(Ag)-AOs with contributions from framework oxygen atoms. Light absorption of Ag14(6+) embedded in the sodalite cage which is characterized by strong peaks centered at 331 and 476 nm (transitions 5s,p(Ag) → 5s,p(Ag)) leads to much longer wavelength emission. The sodalite cage, as a container, stabilizes the central Ag14(6+) cluster by electrostatic attraction. The absorption spectrum of the isovalent neutral Ag8 cluster embedded inside the same sodalite cavity is also simulated using TD-DFT and CASPT2 methods. This absorption spectrum which is similar to that of the Ag14(6+) cluster has two absorption bands in the near UV and visible regions.

Synthesis, structural characterization and high pressure phase transitions of monolithium hydronium sulfate

A three dimensional lithium hydronium sulfate LiSO4·H3O [1], [space group Pna21a=8.7785(12)Å, b=9.1297(12)Å, c=5.2799(7)Å, V=423.16(10)Å3] was synthesized via solvothermal methods using 1,5-naphthalenedisulfonic acid (1,5-NSA) as the source of sulfate ions. The structure of [1], determined by single crystal X-ray diffraction techniques, consists of corner sharing LiO4 and SO4 tetrahedra, forming an anionic 3-D open framework that is charge balanced by hydronium ions positioned within channels running along [001] and forming strong H-bonding with the framework oxygen atoms. Compound [1] undergoes two reversible phase transitions, involving reorientation of SO42− ions at pressures of approximately 2.5 and 5GPa at room temperature, as evident from characteristic discontinuous frequency drops in the ν1 mode of the Raman spectra. Additionally, compound [1] forms dense β-lithium sulfate at 300°C, as evident from temperature dependent powder XRD and combined reversible TGA-DSC experiments.

Using InCl vapor to ion exchange indium into zeolite Na–X. II: A single crystal structure containing (In8Cl8)16+, In57+–Cl−–In57+, and In+

Anhydrous vapor phase ion exchange (VPIE) using InCl(g) had been used to introduce indium ions into zeolite Na92–X (FAU, Si/Al=1.09); the product was In14+(In57+)4Na50+–X. Treatment of Na92–X with a lower vapor pressure of InCl(g) at lower temperatures has now yielded In6.4+(In8Cl8)1.7316+(In10Cl)1.013+Na45.6+–X; Cl− ions were retained and cationic InnClmp+ clusters have formed. Its structure was determined using single-crystal crystallography with synchrotron X-radiation and was refined in the space group Fd3¯ (a=24.890(1)Å) to the final error index R1=0.066. In8Cl816+ clusters center and extend out of 22% of the sodalite cavities. Each consists of a tetrahedrally distorted In4Cl4 cube (In–Cl=2.323(23)Å) with –In–Cl extending radially from each of its Cl− ions (the Cl–In–Cl distances are 2.302(17) and 2.406(14)Å, respectively). With Cl− ions and oxygen atoms of the zeolite framework, these indium ions, all In3+, have octahedral and trigonal bipyramidal coordination, respectively. In57+ cationic clusters center another 25% of the sodalite cavities. In57+ is a centered tetrahedron with In–In=2.745(8)Å; each terminal atom bonds to three framework oxygen atoms of a double 6-ring with In–O=2.150(5)Å and O–In–O=105.1(3)°. One terminal atom of each In57+ cluster also bonds to a Cl− ion at site I, which in turn bonds linearly to a terminal atom of another In57+ cluster. In this way the In57+ units link together in pairs to give In10Cl13+. The remaining indium cations, 6.4 In+ per unit cell, are in the sodalite cavities. Most of the Na+ ions, 25.1 per unit cell, complete the filling of the single 6-rings; the remainder are at 12-ring sites.

Nonclassical hydrides of Ni+ in ZSM-5 zeolite structures: A hybrid DFT study

We have studied the reactivity towards dihydrogen of Ni2+–Ni+ reduced ions, residing above two framework oxygen atoms, using a 18 SiO4 tetrahedrals (18T) model cluster representing the intersection of the straight and the sinusoidal channels in ZSM-5 zeolite. Reported are a detailed analysis of the Ni+–H2 orbital interactions, the thermodynamic stability, and characteristic spectral signatures of the predicted Ni+(H2)n nonclassical hydrides for n=1 and 2, at the B3LYP level of theory. Comparison is made to existing spectroscopy data for a Ni+(H2) complex formed in Ni-exchanged ZSM-5 zeolite [11].

Using InCl Vapor to Ion Exchange Indium into Zeolite Na–X. Single Crystal Structure of |In34Na50|[Si100Al92O384]–FAU Containing In57+ and In+

Indium ions were introduced into zeolite X (FAU, Si/Al = 1.09) by anhydrous vapor-phase ion exchange (VPIE). In14+(In57+)4Na50+–X (approximate formula) was prepared by the reaction of fully dehydrated Na92+–X with 13 Torr of InCl(g) at T = 673 K. Its structure was determined using single-crystal crystallography with synchrotron X-radiation and was refined in the space group Fd3 (a = 24.996(1) A) with all 1566 unique data; the final error index R1 is 0.076. InCl(g) reacted with about 46% of the Na+ ions in the zeolite, to give In57+ (centered tetrahedral, In–In = 2.684(7) A) and In+ cations. In57+ centers exactly half of the sodalite cavities, strongly suggesting alternating occupancy; each terminal atom (at site I′) bonds to three framework oxygen atoms of a double 6-ring (In–O = 2.248(6) A and O–In–O = 102.2(2)°). The In+ ions occupy four crystallographically distinct cationic sites: 5.8(3) per unit cell are in the sodalite cavities (site I′), 2.7(7) are near single 6-rings in the supercage (site II), 2...

Location of MTBE and toluene in the channel system of the zeolite mordenite: Adsorption and host–guest interactions

This paper reports a study of the location of Methyl Tertiary Butyl Ether (MTBE) and toluene molecules adsorbed in the pores of the organophylic zeolite mordenite from an aqueous solution. The presence of these organic molecules in the zeolite channels was revealed by structure refinement performed by the Rietveld method. About 3 molecules of MTBE and 3.6 molecules of toluene per unit cell were incorporated into the cavities of mordenite, representing 75% and 80% of the total absorption capacity of this zeolite. In both cases a water molecule was localized inside the side pocket of mordenite. The saturation capacity determined by the adsorption isotherms, obtained by batch experiments, and the weight loss given by thermogravimetric (TG) analyses were in very good agreement with these values. The interatomic distances obtained after the structural refinements suggest MTBE could be connected to the framework through a water molecule, while toluene could be bonded to framework oxygen atoms. The rapid and high adsorption of these hydrocarbons into the organophylic mordenite zeolite makes this cheap and environmental friendly material a suitable candidate for the removal of these pollutants from water.

Theoretical and experimental investigations on site occupancy for palladium oxidation states in mesoporous Al-MCM-41 materials

The site occupancy of the various oxidation state of palladium supported on mesostructured porous aluminosilica of Al-MCM-41 type was studied combining density functional theory (DFT), FTIR, and Raman investigations. The study focuses on the usual +2 oxidation state highly dispersed during palladium incorporation via template-ion exchange (TIE) but also on other oxidation states. The simulations of the IR spectra using silica clusters of different sizes confirm the Pd2+ preference for sites contained in 6- or 8-membered rings (band at 930cm−1). The calculations suggest that, after calcination, isolated oxidized palladium species such as [PdO]2+, [Pd(OH)2]2+ for the+4 oxidation state, as well as [PdOH]2+ for the+3 EPR active oxidation state may coexist. The match between experimental and calculated vibrational frequencies relies on the two intense Raman bands at 270 and 630cm−1. All these oxidized species are preferentially located on 8-membered rings and linked to three framework oxygen atoms.

A NOVEL POTASSIUM-RICH VARIANT OF TINSLEYITE, K1.5(H2O)0.5 [Al2(OH) (PO4)2

Crystals of a potassium-rich variant of the mineral tinsleyite were obtained by hydrothermal synthesis in the system K 2 O–Al 2 O 3 –P 2 O 5 –H 2 O. The new phase is monoclinic, space group P 2 1 / n , a 9.4352(7), b 9.5254(10), c 9.4309(7) A, β 101.421(7)°, Z = 4. Its crystal structure was solved from single-crystal data collected at 193 K and refined to R = 0.0293, including positions of all hydrogen atoms. The crystal structure is built of units of four AlO 6 octahedra sharing edges and vertices. The octahedron units are cross-linked by PO 4 tetrahedra to form a three-dimensional framework with channels along the main crystallographic [010], [001] and [100] directions, where the large K + ions and H 2 O molecules are distributed. The system of O – H ... O asymmetric hydrogen bonds between H 2 O molecules, hydroxyl groups and framework oxygen atoms provides additional cross-linking in the structure. We show that a change in the alkalinity of the mineral-forming system correlates with the composition of the crystallizing phase and simultaneously influences the character of hydrogen bonding. The new K-rich variety of the tinsleyite is suggested as a possible mineral species. We evaluate the crystal chemistry of the leucophosphite group of minerals and the related synthetic compounds.

Density functional study of copper‐exchanged zeolites and related microporous materials: Adsorption of nitrosyls

AbstractDFT periodic, ONIOM, and cluster studies with all‐electron basis sets are applied to Cu(I) exchanged zeolites and silicoalumino‐phosphate analogs with faujasite and chabazite topology. The reactivity of the cations at different cation positions is probed by NO adsorption. In the ONIOM approach, the cation center and the nearest framework environment are described by DFT, whereas a larger part of the structure is modeled at the semiempirical PM6 level. The importance of including all electrons explicitly in the periodic model computations is outlined by comparison of the results with those from ONIOM and previous plane‐wave studies. The Cu(I) cations reside at distinct extraframework cation sites in the vicinity of the double six‐membered rings (D6R) and upon adsorption they experience a displacement from their previous position. A full optimization with all‐electron basis set is a prerequisite for proper elucidation of the coordination of the transition metal cations by the framework oxygen atoms. © 2012 Wiley Periodicals, Inc.

Lead (II) selenite halides Pb3(SeO3)2 X 2 (X = Br, I): Synthesis and crystal structure

Two lead selenite halides, Pb3(SeO3)2Br2 and Pb3(SeO3)2I2, have been prepared by solid-phase synthesis and structurally characterized. These compounds are isotypic and can be considered 3D with a microporous framework composed of lead polyhedra (distorted Archimedean antiprisms formed by oxygen and halogen atoms). The framework contains channels oriented in the [010] direction. These channels contain selenium atoms, which are bound with framework oxygen atoms belonging to different lead polyhedra.

Basis set effects on Cu(I) coordination in Cu-ZSM-5: a computational study

DFT calculations on the coordination of Cu+ to the framework oxygen atoms of Al-substituted ZSM-5 were performed by using combinations of different basis sets in order to investigate the dependence of the results on the adopted computational level. With low-end basis sets, a large basis set superposition error (BSSE) favors the coordination of Cu+ to three to four oxygen atoms of the framework, only two of which belong to the AlO4 tetrahedron corresponding to the investigated T-site. More extended basis sets considerably lower the BSSE and favor the coordination of Cu+ to only two oxygen atoms of the AlO4 tetrahedron. Upon interaction with NO, the Cu+ ion is always coordinated by two oxygen atoms of the AlO4 tetrahedron, independently of the basis set adopted and of the coordination number before NO adsorption. The shift from three- to twofold coordination caused by the Cu+–NO interaction requires a deformation energy that lowers the final adsorption energy. Such an effect is relevant with low-end basis sets, whereas it is substantially absent with more extended basis sets, which favor the twofold coordination of Cu+ even before NO adsorption. As a result, high-end basis sets increase the NO interaction energy with respect to that calculated by low-end basis sets, in agreement with experiments and suggesting a possible re-interpretation of the catalytic properties of the investigated sites. Provided suitable scale factors are employed, the N–O stretching frequencies of adsorbed nitrogen oxide calculated by sufficiently extended basis sets turned out in fair agreement with experimental findings.

Single-Crystal Structure of a Toluene Sorption Complex of Fully Dehydrated, Fully Mn2+-Exchanged Zeolite Y (FAU), |Mn37.5(C7H8)17|[Si117Al75O384]-FAU

A single crystal of zeolite Y, 0.30 mm in diameter, fully exchanged with Mn2+ and fully dehydrated, was treated with zeolitically dry toluene at 298(1) K and a pressure of 1.3 × 10–4 Pa and evacuated for 2 h. The crystal structure, |Mn37.5(C7H8)17|[Si117Al75O384]-FAU (a = 24.5923(1) Å), was determined by synchrotron X-ray diffraction techniques in the space group Fd3̅m at 100(1) K and was refined to the final error indices R1 = 0.083 and wR2= 0.230. Mn2+ ions occupy sites I, I′, II′, and II with occupancies at 14, 4, 2.5, and 17, respectively. The 17 Mn2+ ions per unit cell at site II each coordinates to three framework oxygen atoms at 2.213(6) Å and extends into the supercage by 0.63 Å from their plane. Each of these Mn2+ ions also interacts facially with a toluene molecule (Mn2+–toluene center = 2.62 Å). The methyl group is somewhat off the plane of the ring, indicative of a net repulsive interaction with the zeolite framework.